A characteristic of a deformable specimen that can be related to area of contact between a mechanical probe and the specimen is the turgor pressure of a cell. Growing plants are hydrostatic structures. Plant form is maintained by turgor pressure. In most of the biomechanics of plant growth, an understanding requires some knowledge of turgor pressure changes to determine the physical properties of the plant, such as yield threshold and wall modulus. However, turgor pressure is not readily measured in a nondestructive, noninvasive way.
Traditional approaches for determining turgor pressure in plant cells were conducted using either an incipient plasmolysis method, a pressure bomb method, or a micropipette-pressure-probe or "micropressure probe" method (see Park S. Nobel, Physicochemical and Environmental Plant Physiology, 103, 176-180, Academic Press Inc., New York, 1991). These traditional methods are laborious and subject to artefactual error. For example, the incipient plasmolysis method is highly subjective, and it radically alters the environment of the cells being measured. The "micropressure probe" method, in contrast, is potentially precise and accurate, but inherently difficult to perform. The micropressure method necessarily destroys the cells whose turgor is being measured. Finally, other techniques, such as the pressure-bomb method, are only suitable for whole organs and are generally characterized as single use, one-shot methods. Thus, the traditional ways of determining turgor pressure are invasive or disruptive to the cellular specimen, thereby interfering with the normal dynamics of the cell, including cellular behavior.
In contrast, the present invention relates to a method and an apparatus for measuring the contact area or contact patch between a specimen and a mechanical probe, and this can be used to determine, virtually instantaneously and repeatably, the turgor pressure in a cellular specimen. The method and apparatus can be non-invasive and non-destructive to the specimen. In cellular specimens the present invention's method can be repeated from point to point, for example, along a growing axis.